Hi,
sorry I don't know, this is a SQL problem caused by the change of DB collation.

I have the same probleme before i changed the collation of all tables.

In fact, the error occurs when the username contains accents and i want to view the profil of this user.

any ideas to resolve this ?

Hi,
I think the error come from a query that search ID from Username in file /components/com_jsn/router.php at line 142.

In this line there is a simple query to standard Joomla table (#__users), and in all environments it work, so I think this is a problem of your DB.

To check this we need to access to your DB with some tool like phpMyAdmin (you can use the form below the reply in "Site Details" tab).

Site details here

After many tests, also by changing charset in my dev site we can't replicate this problem.
After some investigation I found that when you change column/table/db charset then you change only default charset if db server do not know value charset.

Seems that you have imported this table with a old charset not know by your server but compatible with utf8_general_ci and utf8_unicode_ci (maybe it is utf_general_ci).

To solve you can try to export #__users table, check charset in table declaration (CREATE TABLE.....) and reimport.
or Alternative: we can try to solve this problem by changing something in router php, but we need to have ftp access because we can't replicate this problem.